1. Field of the Invention
The present invention relates to a capacitor that constitutes DRAM and a process for manufacturing the same, in particular, to a capacitor structure suitable for eliminating the problem of the collapse of the lower electrode, which is caused when forming a lower electrode having a crown shape, and a process for manufacturing the same.
2. Description of the Prior Art
In recent years the capacity of semiconductor devices has been increased more and more. Particularly in DRAMs (DRAM: dynamic random access memory), gigabit-class memories with a minimum feature size of 100 nm are being commercialized, and moreover, development of DRAMs with a minimum feature size of 90 nm or smaller are being proceeded with. With such miniaturization of semiconductor devices, it has become difficult to ensure desired capacity of capacitors, which are principal constituents of DRAMs.
To overcome this difficulty, a capacitor having a crown structure has been examined, in which a trench (deep hole) is formed in an insulating film, both the inside and outside walls of a lower electrode formed on the inside face of the trench are exposed, and the both sides thereof are used as a capacitor. In the capacitor having a crown structure, it is possible to ensure capacitor area about two times larger than that of a capacitor in which only the inside face of the trench is used. Accordingly, the capacitor having a crown structure offers the advantage that it has capacity two times higher than that of a capacitor in which a lower electrode is provided only on the inside face of the trench.
However, conventional processes for preparing a crown-structure capacitor present the problems described below. FIGS. 1A to 1C schematically show the manufacturing process of a crown-structure capacitor. First, as shown in FIG. 1A, a silicon plug 103 is formed in a specified region of a first inter layer dielectric 101, and a silicon nitride film 102 and a second inter layer dielectric 104 consisting of a thick silicon oxide film are deposited. Then, as shown in FIG. 1B, a trench 105 is formed by lithography and dry etching to expose the surface of the silicon plug 103 and then a lower electrode 106 is formed on the inside face of trench. After that, as shown in FIG. 1C, the second inter layer dielectric 104 which supports and surrounds the outside walls of the lower electrode 106 is removed using a hydrofluoric acid (HF) solution. Once the thick silicon oxide film is removed using the HF solution, the lower electrode 106 loses its support and its mechanical strength significantly decreases. As a result, the lower electrode 106 collapses due to the surface tension of the HF solution, causing a pair bit defect, because the adjacent lower electrodes are brought into contact with each other. If the silicon oxide film can be removed by dry etching, which causes no surface tension, the collapse of the lower electrode 106 is effectively prevented. However, in the present state, no practical technique has been realized yet which makes it possible to remove only the silicon oxide film without damaging the shape of the lower electrode.
Under these conditions, a crown-structure capacitor is thought of in which the capacitance is increased without removing the second inter layer dielectric. This technique is described in Japanese Patent Application Laid-Open No. 10-173148. FIGS. 2A to 2G show the manufacturing process of a crown-structure capacitor which is described in Example of the above described patent specification. In the following, the manufacturing process of the crown-structure capacitor described in the patent specification will be explained with reference to FIGS. 2A to 2G.
First, as shown in FIG. 2A, a silicon plug 103 is formed in a specified region of a first inter layer dielectric 101, a silicon nitride film 102 and a second inter layer dielectric 104 consisting of a thick silicon oxide film are deposited. A trench 105 is formed so that the surface of the silicon plug 103 is exposed, and after that a first upper electrode 107 made of polycrystalline silicon is formed on the side wall of the trench 105.
Then, as shown in FIG. 2B, a first dielectric 108 consisting of stacked film made of tantalum oxide and silicon oxide is deposited on the whole surface and then a outside lower electrode 109 made of titanium nitride is deposited and stacked on the whole surface. After that, the outside lower electrode 109 and the first dielectric 108, which are formed on the surface other than the surface of the trench and on the trench bottom, are removed using anisotropic dry etching.
Then, as shown in FIG. 2C, an inside lower electrode 110 made of polycrystalline silicon is deposited and the inside of the trench is filled with a photoresist 111. The photoresist 111 is formed so that its top surface is positioned a little lower than the top of the trench.
Then, as shown in FIG. 2D, the inside lower electrode 110 made of polycrystalline silicon and the outside lower electrode 109 made of titanium nitride are subjected to dry etching so that each top surface is almost at the same level as the top surface of the photoresist 111.
Then, as shown in FIG. 2E, the photoresist 111 is removed, a second dielectric 112 consisting of a stacked film made of tantalum oxide and silicon oxide is deposited, and a second upper electrode 113 made of titanium nitride is deposited on the whole surface so that the trench is filled up with the electrode, followed by etching back so that the top surface of the second upper electrode 113 is at the level shown in the figure.
Then, as shown in FIG. 2F, the exposed portion of the second dielectric 112 is subjected to dry etching so that the top surface of the second dielectric 112 is almost at the same level as the top surface of the second upper electrode 113. At this time, the first dielectric 108 undergoes etching simultaneously, whereby the top of the first upper electrode 107 is exposed.
Then, as shown in FIG. 2G, a third upper electrode 114 made of titanium nitride is deposited on the whole surface so that the first upper electrode 107 and the second upper electrode 113 are connected with each other. The lower electrode connected with the silicon plug 103 is constructed by the inside lower electrode 110 made of polycrystalline silicon and the outside lower electrode 109 made of titanium nitride. Further, the first dielectric 108 consisting of a stacked film made of tantalum oxide and silicon oxide is provided between the first upper electrode 107 and the outside lower electrode 109, and the second dielectric 112 consisting of a stacked film made of tantalum oxide and silicon oxide is provided between the second upper electrode 113 and the inside lower electrode 110, whereby a capacitor having a crown structure is made up in the inside of the trench.
This known example offers the advantage of being capable of preventing the collapse of its lower electrode, because the insulating film constituting the trench is not removed.
However, by the manufacturing process of the crown-structure capacitor described in Japanese Patent Application Laid-Open No. 10-173148, it is very difficult to connect the first upper electrode 107, which is formed on the inside wall of the trench, with the third upper electrode 114. This presents the problem of being unable to make up a crown structure.
In the following, the problem will be described with reference to FIGS. 3A to 3D.
FIG. 3A shows the state after the photoresist 111, which is filled into the inside of the trench after deposition of the inside lower electrode 110 made of polycrystalline silicon in the step shown in FIG. 2C, has been removed by etching back employing dry etching. Etching back is intended to selectively etch the outside lower electrode 109 made of titanium nitride and the inside lower electrode 110 made of polycrystalline silicon so that their top surfaces are made lower as shown in FIG. 2D. However, in actuality, the first upper electrode 107 is unintentionally etched together and its top surface is also made lower. Naturally, the etching progresses on both the first upper electrode 107 and the inside lower electrode 110 since the first upper electrode 107 is made of polycrystalline silicon just like the inside lower electrode 110. As a result, the first dielectric 108 projects from the top surface, forming a vacant space 115 over the first upper electrode 107.
Then, as shown in FIG. 3B, when the second dielectric 112 is deposited, the vacant space 115 is filled with the second dielectric 112, resulting in the formation of an insulating film on the top surface of the first upper electrode 107. The inside upper electrode 113 is formed in this state.
Then, as shown in FIG. 3C, even if the second dielectric 112 is subjected to etch back, the second dielectric 116, which is the second dielectric 112 remaining in the vacant space 115, is formed on the first upper electrode 107, whereby the top surface of the first upper electrode 107 is not exposed.
Even if the third upper electrode 114 is formed in the above state, as shown in FIG. 3D, the connection between the third upper electrode 114 and the first upper electrode 107 can not be ensured because the top surface of the first upper electrode 107 is covered with the second dielectric 116, whereby a crown-structure capacitor can not be realized.
As described so far, when intending to form a crown-structure capacitor in the inside of a trench, the biggest technical problem is to ensure the connection between the third upper electrode 114 and the first upper electrode 107.
In the light of the problem described above, the object of the present invention is to provide a capacitor having a crown structure which aims at electrical connection between upper electrodes by easier and simpler method utilizing an insulating property of tantalum oxide and a process for manufacturing the same.